The formation of slimes by microorganisms is a problem that is encountered in many aqueous systems. For example, the problem is not only found in natural waters such as lagoons, lakes, ponds, etc., and confined waters as in pools, but also in such industrial systems as cooling water systems, air washer systems, secondary/tertiary oil production operations, and pulp and paper mill systems. All possess conditions which are conducive to the growth and reproduction of slime-forming organisms. In both once-through and recirculating cooling systems, for example, which employ large quantities of water as a cooling medium, the formation of slime by microorganisms is an extensive and constant problem. Furthermore, the slime can also support the growth of other microbes as part of the environment. The possibility exists that some of these organisms are pathogens such as Legionella pneumophila. 
Airborne organisms are readily entrained in the water from cooling towers and find this warm medium an ideal environment for growth and replication. Aerobic and heliotropic organisms flourish on the tower deck and other tower surfaces proper, whereas others circulate in the bulk water throughout the system, attach and grow in areas such as the sump, piping, and passages of the cooling system. The slime formation not only aids in the deterioration of the tower structure, but also, acts as the source of microbes that can break off, deposit on metal surfaces, and form biofilms promoting corrosion. Slime carried through the cooling system plugs and fouls lines, valves, strainers, etc. and deposits on heat exchange surfaces. In the latter case, the impedance of the heat transfer can greatly reduce the efficiency of the cooling system.
In pulp and paper mill systems, slime formed by microorganisms is commonly encountered and causes fouling, plugging, or corrosion of the system. The slime also becomes entrained in the paper produced to cause breakouts on the paper machines, which results in work stoppages and the loss of production time. The slime is also responsible for unsightly blemishes in the final product, which result in rejects and wasted output.
In secondary/tertiary oil recovery operations where water flooding is employed, slime forming microbes on pipe surfaces can host the attachment and protection of other organisms such as sulfate reducers and other anaerobes causing corrosion resulting in degradation and leaks of pipelines.
The previously discussed problems have resulted in the extensive utilization of biocides in cooling water and pulp and paper mill systems. Materials which have enjoyed widespread use in such applications include chlorine, chlorinated phenols, bromine compounds, organo-bromines, and various organo-sulfur compounds. All of these compounds are generally useful for this purpose especially against bacterial microorganisms, but each is attended by a variety of impediments. For example, chlorination is limited both by its specific toxicity for slime forming organisms at economic levels and by the tendency of chlorine to react non-specifically with organic material resulting in the expenditure of the chlorine before its full biocidal function is achieved. Often the thickness of a well-established slime biofilm inhibits chlorine from penetrating the biofilm, so only the surface organisms are destroyed, allowing for quick recovery and growth. The sulfate reducing bacteria are not affected and the corrosion rates will continue. Other biocides are attended by odor problems and hazards in respect to storage, use, or handling which limit their utility. To date, no one compound or type of compound has achieved a clearly established predominance with respect to the applications discussed. Likewise, lagoons, ponds, lakes, and even pools, either used for pleasure purposes or used for industrial purposes for disposal and storage of industrial wastes, become, during the warm weather, besieged by slime due to microorganism growth and reproduction. In the case of the recreational areas, the problem of infection is obvious. In the case of storage or disposal of industrial materials, the microorganisms cause additional problems which must be eliminated prior to the material""ss use or disposal of the waste.
Naturally, economy is a major consideration with respect to all of these biocides. Such economic considerations are attached to both the cost of the biocide and the expense of its application. The cost performance index of any biocide is derived from the basic cost of the material, its effectiveness per unit weight, the duration of its biocidal or biostatic effect on the system treated, and the ease and frequency of its addition to the system treated. To date, none of the commercially available biocides has exhibited a prolonged biocidal effect. Instead, their effectiveness is rapidly reduced as the result of exposure to physical conditions such as temperature, association with ingredients contained by the system toward which they exhibit an affinity or substantivity, pH, etc., with a resultant restriction or elimination of their biocidal effectiveness, or by dilution.
As a consequence, the use of such biocides involves their continuous or frequent addition to systems to be treated and their addition to multiple points or zones in the systems to be treated. Accordingly, the cost of the biocide and the labor costs associated with such means of applying it are considerable. In other instances, the difficulty of access to a zone or entire zone in which slime formation is experienced precludes the effective use of a biocide. For example, if in a particular system there is no access to an area at which slime formation occurs such as tower fill, the biocide can only be applied at a point which is upstream in the flow system. The physical or chemical conditions, e.g., chemical reactivity, thermal degradation, etc., which exists between the point where the biocide may be added to the system and the point at which its biocidal effect is desired, sometimes render the effective use of a biocide impossible.
Similarly, in a system experiencing relatively slow flow, such as a paper mill, if a biocide is added at the beginning of the system, its biocidal effect may be completely dissipated before it has reached all of the points at which this effect is desired or required. As a consequence, the biocide must be added at multiple points, and even then a diminishing biocidal effect will be experienced between one point of addition to the system and the next point downstream at which the biocides may be added. In addition to the increased cost of utilizing and maintaining multiple feed points, gross ineconomies with respect to the cost of the biocide are experienced. Specifically, at each point of addition, an excess of the biocide is added to the system in order to compensate for that portion of the biocide which will be expended in reacting with other constituents present in the system or experience physical changes which impair its biocidal activity.
The biocidal compositions of the present invention comprise, as active ingredients, 1) glutaraldehyde and 2) 2,2-dibromo-3-nitrilopropionamide (DBNPA). These constituents are commercially available. The synergistic effect obtained by combining glutaraldehyde and DBNPA has not been previously disclosed.